1. Field of Invention
The invention relates to a hydraulic control unit for an automatic transmission.
2. Description of Related Art
Conventional automatic transmissions used in automobiles and other vehicles are equipped with a transmission device provided with a planetary gear unit and other parts. Such a transmission device is provided with a plurality of friction engagement elements including a clutch and a brake and, making each of these friction engagement elements engage or disengage in a predetermined combination causes an output of rotation to be produced selectively from gear elements, such as a sun gear, ring gear, and carrier of the planetary gear unit to achieve a plurality of gearshift positions.
Each of the friction engagement elements is engaged or disengaged by operating a hydraulic servo of a hydraulic circuit. The hydraulic servo is provided with a hydraulic servo drum having an outer peripheral wall and an inner peripheral wall, and an annular piston fitted slidably into the hydraulic servo drum, etc. By supplying oil to, and discharging it from, an application oil chamber formed between the hydraulic servo drum and the annular piston, the annular piston is moved, thereby engaging or disengaging a friction engagement element disposed to be opposed to the annular piston.
Also, solenoid valves and selector valves, etc. of various kinds are disposed in the hydraulic circuit. When the solenoid of a predetermined solenoid valve is turned ON or OFF, a predetermined selector valve is selected and hydraulic pressure is supplied to, or exhausted from, a hydraulic servo connected to the corresponding selector valve.
However, the automatic transmission of such a configuration is designed to cause one or more specific friction engagement elements to be engaged to achieve a certain gearshift position. If, however, a solenoid valve fails and a friction engagement element that should not be engaged is engaged, then interlock occurs in the transmission device.
A hydraulic circuit is therefore provided in which, if a hydraulic pressure is about to be supplied to a hydraulic servo of a friction engagement element which should not be engaged, that hydraulic pressure is used to select a predetermined selector valve, thereby preventing the hydraulic pressure from being supplied to the hydraulic servo and thus preventing interlock from occurring in the transmission device (See Japanese Patent Application Laid-Open Publication SHO 63-210443).
In the conventional automatic transmission, however, interlock can be prevented from occurring in the transmission device in a condition in which a friction engagement element that should not be engaged is engaged, that is, a condition in which a failure has occurred due to hydraulic pressure being supplied to the hydraulic servo. However, in a condition in which a friction engagement element that should be engaged is not engaged, that is, in a condition in which failure has occurred due to hydraulic pressure not being supplied to the hydraulic servo, a neutral condition is established in the transmission device depending on the type of the automatic transmission, and therefore power drive cannot be transmitted via the transmission device. This could result in not only a desired gearshift not being performed properly, but also the vehicle not being able to be moved.
In addition, depending on the solenoid valve that has failed, a gearshift from a high-speed gear position such as 4th speed or 5th speed, to a low-speed gear position such as 1st speed or 2nd speed, is made, thus applying large engine braking and generating gearshift shock.
To overcome the problems of the conventional automatic transmission, the invention provides a hydraulic control unit for an automatic transmission that can prevent interlock from occurring in the transmission device, prevent a neutral condition from being established in the transmission device, prevent a gearshift from a high-speed gear position to a low-speed gear position from being made, and prevent gearshift shock from being generated.
To achieve the foregoing, a hydraulic control unit for an automatic transmission according to an exemplary aspect of the invention is provided with hydraulic servos, each of which engages and disengages respective friction engagement elements, hydraulic pressure supply means which supplies hydraulic pressure to a hydraulic servo of a predetermined friction engagement element selected according to a gearshift position to be achieved, failure detection means that detects that a failure has occurred in the hydraulic pressure supply means, and fail-safe means that prevents interlock from occurring due to a plurality of friction engagement elements engaging, and prevents a shift from a high-speed gear position to a low-speed gear position when the failure detection means detects that a failure has occurred.
In this case, when the failure detection means detects that a failure has occurred in a hydraulic pressure supply means, interlock occurring as a result of a plurality of friction engagement elements being engaged is prevented from occurring. A predetermined gearshift position on the high-speed position side is then achieved.
Accordingly, since gearshifting from a high-speed position to a low-speed position is prevented along with the prevention of the occurrence of interlock, a large engine braking being applied is prevented. As a result, the generation of gearshift shock is prevented.
Another hydraulic control unit for an automatic transmission according to another exemplary aspect of the invention is provided with hydraulic servos, each of which engages and disengages respective friction engagement elements, hydraulic pressure supply means which supplies hydraulic pressure to a hydraulic servo of a predetermined friction engagement element selected according to a gearshift position to be achieved, failure detection means that detects that a failure has occurred in the hydraulic pressure supply means, and fail-safe means that prevents a neutral condition from being established, and prevents a gearshift from a high-speed gear position to a low-speed gear position when the failure detection means detects that a failure has occurred.
In this case, when the failure detection means detects that a failure has occurred in a hydraulic pressure supply means, the establishment of a neutral condition is prevented. A predetermined gearshift position on the high-speed position side is then achieved.
Accordingly, since gearshifting from a high-speed position to a low-speed position is prevented along with the prevention of the establishment of a neutral condition, a large engine braking being applied is prevented. As a result, the generation of gearshift shock is prevented.
Still another hydraulic control unit for an automatic transmission according to another exemplary aspect of the invention is provided with hydraulic servos, each of which engages and disengages respective friction engagement elements, hydraulic pressure supply means which supplies hydraulic pressure to a hydraulic servo of a predetermined friction engagement element selected according to a gearshift position to be achieved, failure detection means that detects that a failure has occurred in the hydraulic pressure supply means, and fail-safe means that prevents interlock from occurring due to a plurality of friction engagement elements engaging, prevents a neutral condition from being established and prevents a gearshift from a high-speed gear position to a low-speed gear position when the failure detection means detects that a failure has occurred.
In this case, when the failure detection means detects that a failure has occurred in a hydraulic pressure supply means, interlock occurring as a result of a plurality of friction engagement elements being engaged is prevented from occurring and a neutral condition is prevented from being established. A predetermined gearshift position on the high-speed position side is then achieved.
Accordingly, since gearshifting from a high-speed position to a low-speed position is prevented along with the prevention of the occurrence of interlock and the establishment of a neutral condition, a large engine braking being applied is prevented. As a result, the generation of gearshift shock is prevented.
In a further hydraulic control unit for an automatic transmission according to another exemplary aspect of the invention, the failure detection means is a failure detection valve that is provided with hydraulic pressure detection portions that detect that hydraulic pressures supplied to a plurality of hydraulic servos are generated simultaneously, and an output portion that generates a fail-safe pressure indicating that a failure has occurred when hydraulic pressures supplied to a plurality of hydraulic servos has been generated simultaneously, and outputs the fail-safe pressure to the fail-safe means.
In a still further hydraulic control unit for an automatic transmission according to another exemplary aspect of the invention, the failure detection means detects that hydraulic pressures supplied to each of the hydraulic servos for two clutches and one brake are generated simultaneously.
In a yet further hydraulic control unit for an automatic transmission according to another exemplary aspect of the invention, the fail-safe means is provided with a selector valve that, according to a selection, takes a first selector position at which a gear ratio of a sub-transmission is made larger and a second selector position at which the gear ratio of the sub-transmission is made smaller, and causes the selector valve to switch to the second selector position when the fail-safe pressure is supplied.
In a yet further hydraulic control unit for an automatic transmission according to another exemplary aspect of the invention, the selector valve allows a vehicle to drive in reverse when in the first selector position and prevents the vehicle from driving in reverse when in the second selector position.
In a yet further hydraulic control unit for an automatic transmission according to another exemplary aspect of the invention, the fail-safe means is provided with a first selector valve that, according to selection, supplies a hydraulic pressure to each of the hydraulic servos of the two clutches and supplies or exhausts a hydraulic pressure to or from the hydraulic servo of one brake, and a second selector valve that, according to the selection, takes a first selector position at which a gear ratio of a sub-transmission is made larger and a second selector position at which the gear ratio of the sub-transmission is made smaller, and causes the first selector valve to switch such that a hydraulic pressure is supplied to each of the hydraulic servos of the two clutches and the second selector valve to switch to the second position when the fail-safe pressure is supplied.
In a yet further hydraulic control unit for an automatic transmission according to another exemplary aspect of the invention, the second selector valve allows a vehicle to drive in reverse when in the first selector position and prevents the vehicle from driving in reverse when in the second selector position.